Climate models overheat Antarctica, new study finds

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BOULDER--Computer analyses of global climate have consistently overstated warming in Antarctica, concludes new research by scientists at the National Center for Atmospheric Research (NCAR) and Ohio State University. The study can help scientists improve computer models and determine if Earth's southernmost continent will warm significantly this century, a major research question because of Antarctica's potential impact on global sea-level rise.
"We can now compare computer simulations with observations of actual climate trends in Antarctica," says NCAR scientist Andrew Monaghan, the lead author of the study. "This is showing us that, over the past century, most of Antarctica has not undergone the fairly dramatic warming that has affected the rest of the globe. The challenges of studying climate in this remote environment make it difficult to say what the future holds for Antarctica's climate."

BOULDER--Computer analyses of global climate have consistently
overstated warming in Antarctica, concludes new research by scientists
at the National Center for Atmospheric Research (NCAR) and Ohio State
University. The study can help scientists improve computer models and
determine if Earth's southernmost continent will warm significantly
this century, a major research question because of Antarctica's
potential impact on global sea-level rise.

"We can now compare
computer simulations with observations of actual climate trends in
Antarctica," says NCAR scientist Andrew Monaghan, the lead author of
the study. "This is showing us that, over the past century, most of
Antarctica has not undergone the fairly dramatic warming that has
affected the rest of the globe. The challenges of studying climate in
this remote environment make it difficult to say what the future holds
for Antarctica's climate."

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The study marks the first time that
scientists have been able to compare records of the past 50 to 100
years of Antarctic climate with simulations run on computer models.
Researchers have used atmospheric observations to confirm that computer
models are accurately simulating climate for the other six continents.
The models, which are mathematical representations of Earth's climate
system, are a primary method for scientists to project future climate.

Antarctica's
climate is of worldwide interest, in part because of the enormous water
locked up in its ice sheets. If those vast ice sheets were to begin to
melt, sea level could rise across the globe and inundate low-lying
coastal areas. Yet, whereas climate models accurately simulate the last
century of warming for the rest of the world, they have unique
challenges simulating Antarctic climate because of limited information
about the continent's harsh weather patterns.

The study was
published on April 5 in Geophysical Research Letters. It was funded by
the National Science Foundation, NCAR's primary sponsor, and the
Department of Energy.

The authors compared recently
constructed temperature data sets from Antarctica, based on data from
ice cores and ground weather stations, to 20th century simulations from
computer models used by scientists to simulate global climate. While
the observed Antarctic temperatures rose by about 0.4 degrees
Fahrenheit (0.2 degrees Celsius) over the past century, the climate
models simulated increases in Antarctic temperatures during the same
period of 1.4 degrees F (0.75 degrees C).

The error appeared
to be caused by models overestimating the amount of water vapor in the
Antarctic atmosphere, the new study concludes. The reason may have to
do with the cold Antarctic atmosphere handling moisture differently
than the atmosphere over warmer regions.

The models, however,
have correctly captured trends in Antarctic snowfall, including
increases in snowfall in the late 20th century, prior to a decrease
over the last decade.

-----A chilling ozone hole-----

Part
of the reason that Antarctica has barely warmed has to do with the
ozone hole over the continent. The lack of ozone is chilling the middle
and upper atmosphere, altering wind patterns in a way that keeps
comparatively warm air from reaching the surface. Unlike the rest of
the continent, the Antarctic Peninsula has warmed by several degrees,
in part because the winds there are drawing in warmer air from the
north. The models generally capture these wind changes, although
sometimes incompletely.

The study delivered a mixed verdict on
Antarctica's potential impact on sea-level rise. The Intergovernmental
Panel on Climate Change, which operates under the auspices of the
United Nations, has estimated that sea-level rise could amount to 7 to
23 inches (18-59 centimeters) this century, in part because of melting
glaciers worldwide. The Geophysical Research Letters paper suggests
that warming in Antarctica over the next century could offset that by
about 2 inches if the continent warms by 5.4 degrees F (3 degrees C),
as computer models have indicated. The reason is that the warmer air
over Antarctica would hold more moisture and generate more snowfall,
thereby locking up additional water in the continent's ice sheets.

But the authors caution that model projections of future Antarctic climate may be unreliable.

"The
research clearly shows that you can actually slow down sea-level rise
when you increase temperatures over Antarctica because snowfall
increases, but warmer temperatures also have the potential to speed up
sea-level rise due to enhanced melting along the edges of Antarctica,"
says Monaghan, who did some of his research at Ohio State University
before coming to NCAR. "Over the next century, whether the ice sheet
grows from increased snowfall or shrinks due to more melt will depend
on how much temperatures increase in Antarctica, and potentially on
erosion at the ice sheet edge by the warmer ocean and rising sea level."

"The
current generation of climate models has improved over previous
generations, but still leaves Antarctic surface temperature projections
for the 21st century with a high degree of uncertainty," adds co-author
and NCAR scientist David Schneider. "On a positive note, this study
points out that water vapor appears to be the key cause of the
problematic Antarctic temperature trends in the models, which will
guide scientists as they work to improve the climate simulations."